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Cellphones are not just a public nuisance—they are also used by
lookouts to warn criminals when the police are near. Now Deropa of Boksburg in
South Africa is patenting a transmitter that jams cellphones (WO 98/18232). All
cellphones continually transmit and receive coded signals, with which they
communicate with the network base stations. If these signals cannot get through,
the phone cannot make or receive calls. By broadcasting low-power noise, the
transmitter blocks these signals, preventing the cellphone from working.